Well you know the MG-Rover brand more than most, so that's a start
The MR2 is a great drivers car, with handling on a par to the TF, and I wouldn't have hesitated to have one. In contest with the TF though, I found that I like the looks of the TF more than both the MX5 or MR2, and also found the MR2 to be far less practical as a daily drive. I haven't driven an MX5, so can't comment there, but having enjoyed the exclusivity of the TF I would hate to fade into the background as "yet another Mazda" owner!
The bad reputation stems largely from head gasket issues, which you're aware of as a Rover owner. As long as you are prepared for it and know what measures to take to prevent it then you're off to a flying start. Picking up an example that has been well-cared for and maintained is a must.
If someone really wants into your TF then a softop or hardtop makes no difference - they'll get in by breaking a window if required. Higher spec cars have a volumetric alarm which can detect intrusion into the passenger cabin, but lower spec versions (e.g. our 115) lack this feature.
Our cars might be an easy target for slating, but I don't think there's a person out there who wouldn't have a grin on their face after driving one, especially after realising how little money is required to get hold of a great example. I'm convinced there is a degree of bitterness from those who desperately pay top whack to label themselves as "urban" and join the MPV/4x4/Germanic/crossover depreciation conveyor belt, only for us to turn up, smug and self-satisfied, in our dated, dead branded, cheap, unreliable, roofless, beautiful machines.